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<chapter xmlns="http://docbook.org/ns/docbook"
xmlns:xlink="http://www.w3.org/1999/xlink"
xmlns:xi="http://www.w3.org/2001/XInclude"
version="5.0"
xml:id="sec-common-env">
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<title>Common Environment Variables</title>
<para>Most Nix commands interpret the following environment variables:</para>
<variablelist xml:id="env-common">
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<varlistentry><term><envar>IN_NIX_SHELL</envar></term>
<listitem><para>Indicator that tells if the current environment was set up by
<command>nix-shell</command>.</para></listitem>
</varlistentry>
<varlistentry xml:id="env-NIX_PATH"><term><envar>NIX_PATH</envar></term>
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<listitem>
<para>A colon-separated list of directories used to look up Nix
expressions enclosed in angle brackets (i.e.,
<literal>&lt;<replaceable>path</replaceable>></literal>). For
instance, the value
<screen>
/home/eelco/Dev:/etc/nixos</screen>
will cause Nix to look for paths relative to
<filename>/home/eelco/Dev</filename> and
<filename>/etc/nixos</filename>, in that order. It is also
possible to match paths against a prefix. For example, the value
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<screen>
nixpkgs=/home/eelco/Dev/nixpkgs-branch:/etc/nixos</screen>
will cause Nix to search for
<literal>&lt;nixpkgs/<replaceable>path</replaceable>></literal> in
<filename>/home/eelco/Dev/nixpkgs-branch/<replaceable>path</replaceable></filename>
and
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<filename>/etc/nixos/nixpkgs/<replaceable>path</replaceable></filename>.</para>
<para>If a path in the Nix search path starts with
<literal>http://</literal> or <literal>https://</literal>, it is
interpreted as the URL of a tarball that will be downloaded and
unpacked to a temporary location. The tarball must consist of a
single top-level directory. For example, setting
<envar>NIX_PATH</envar> to
<screen>
nixpkgs=https://github.com/NixOS/nixpkgs-channels/archive/nixos-14.12.tar.gz</screen>
tells Nix to download the latest revision in the Nixpkgs/NixOS
14.12 channel.</para>
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<para>The search path can be extended using the <option
linkend="opt-I">-I</option> option, which takes precedence over
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<envar>NIX_PATH</envar>.</para></listitem>
</varlistentry>
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<varlistentry><term><envar>NIX_IGNORE_SYMLINK_STORE</envar></term>
<listitem>
<para>Normally, the Nix store directory (typically
<filename>/nix/store</filename>) is not allowed to contain any
symlink components. This is to prevent “impure” builds. Builders
sometimes “canonicalise” paths by resolving all symlink components.
Thus, builds on different machines (with
<filename>/nix/store</filename> resolving to different locations)
could yield different results. This is generally not a problem,
except when builds are deployed to machines where
<filename>/nix/store</filename> resolves differently. If you are
sure that youre not going to do that, you can set
<envar>NIX_IGNORE_SYMLINK_STORE</envar> to <envar>1</envar>.</para>
<para>Note that if youre symlinking the Nix store so that you can
put it on another file system than the root file system, on Linux
youre better off using <literal>bind</literal> mount points, e.g.,
<screen>
$ mkdir /nix
$ mount -o bind /mnt/otherdisk/nix /nix</screen>
Consult the <citerefentry><refentrytitle>mount</refentrytitle>
<manvolnum>8</manvolnum></citerefentry> manual page for details.</para>
</listitem>
</varlistentry>
<varlistentry><term><envar>NIX_STORE_DIR</envar></term>
<listitem><para>Overrides the location of the Nix store (default
<filename><replaceable>prefix</replaceable>/store</filename>).</para></listitem>
</varlistentry>
<varlistentry><term><envar>NIX_DATA_DIR</envar></term>
<listitem><para>Overrides the location of the Nix static data
directory (default
<filename><replaceable>prefix</replaceable>/share</filename>).</para></listitem>
</varlistentry>
<varlistentry><term><envar>NIX_LOG_DIR</envar></term>
<listitem><para>Overrides the location of the Nix log directory
(default <filename><replaceable>prefix</replaceable>/log/nix</filename>).</para></listitem>
</varlistentry>
<varlistentry><term><envar>NIX_STATE_DIR</envar></term>
<listitem><para>Overrides the location of the Nix state directory
(default <filename><replaceable>prefix</replaceable>/var/nix</filename>).</para></listitem>
</varlistentry>
<varlistentry><term><envar>NIX_DB_DIR</envar></term>
<listitem><para>Overrides the location of the Nix database (default
<filename><replaceable>$NIX_STATE_DIR</replaceable>/db</filename>, i.e.,
<filename><replaceable>prefix</replaceable>/var/nix/db</filename>).</para></listitem>
</varlistentry>
<varlistentry><term><envar>NIX_CONF_DIR</envar></term>
<listitem><para>Overrides the location of the Nix configuration
directory (default
<filename><replaceable>prefix</replaceable>/etc/nix</filename>).</para></listitem>
</varlistentry>
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<varlistentry><term><envar>TMPDIR</envar></term>
<listitem><para>Use the specified directory to store temporary
files. In particular, this includes temporary build directories;
these can take up substantial amounts of disk space. The default is
<filename>/tmp</filename>.</para></listitem>
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</varlistentry>
<varlistentry xml:id="envar-build-hook"><term><envar>NIX_BUILD_HOOK</envar></term>
<listitem>
<para>Specifies the location of the <emphasis>build hook</emphasis>,
which is a program (typically some script) that Nix will call
whenever it wants to build a derivation. This is used to implement
distributed builds<phrase condition="manual"> (see <xref
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linkend="chap-distributed-builds" />)</phrase>.</para>
<!--
The protocol by
which the calling Nix process and the build hook communicate is as
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follows.
<para>The build hook is called with the following command-line
arguments:
<orderedlist>
<listitem><para>A boolean value <literal>0</literal> or
<literal>1</literal> specifying whether Nix can locally execute
more builds, as per the <link
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linkend="opt-max-jobs"><option>- -max-jobs</option> option</link>.
The purpose of this argument is to allow the hook to not have to
maintain bookkeeping for the local machine.</para></listitem>
<listitem><para>The Nix platform identifier for the local machine
(e.g., <literal>i686-linux</literal>).</para></listitem>
<listitem><para>The Nix platform identifier for the derivation,
i.e., its <link linkend="attr-system"><varname>system</varname>
attribute</link>.</para></listitem>
<listitem><para>The store path of the derivation.</para></listitem>
</orderedlist>
</para>
<para>On the basis of this information, and whatever persistent
state the build hook keeps about other machines and their current
load, it has to decide what to do with the build. It should print
out on standard error one of the following responses (terminated by
a newline, <literal>"\n"</literal>):
<variablelist>
<varlistentry><term><literal># decline</literal></term>
<listitem><para>The build hook is not willing or able to perform
the build; the calling Nix process should do the build itself,
if possible.</para></listitem>
</varlistentry>
<varlistentry><term><literal># postpone</literal></term>
<listitem><para>The build hook cannot perform the build now, but
can do so in the future (e.g., because all available build slots
on remote machines are in use). The calling Nix process should
postpone this build until at least one currently running build
has terminated.</para></listitem>
</varlistentry>
<varlistentry><term><literal># accept</literal></term>
<listitem><para>The build hook has accepted the
build.</para></listitem>
</varlistentry>
</variablelist>
</para>
<para>After sending <literal># accept</literal>, the hook should
read one line from standard input, which will be the string
<literal>okay</literal>. It can then proceed with the build.
Before sending <literal>okay</literal>, Nix will store in the hooks
current directory a number of text files that contain information
about the derivation:
<variablelist>
<varlistentry><term><filename>inputs</filename></term>
<listitem><para>The set of store paths that are inputs to the
build process (one per line). These have to be copied
<emphasis>to</emphasis> the remote machine (in addition to the
store derivation itself).</para></listitem>
</varlistentry>
<varlistentry><term><filename>outputs</filename></term>
<listitem><para>The set of store paths that are outputs of the
derivation (one per line). These have to be copied
<emphasis>from</emphasis> the remote machine if the build
succeeds.</para></listitem>
</varlistentry>
<varlistentry><term><filename>references</filename></term>
<listitem><para>The reference graph of the inputs, in the format
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accepted by the command <command>nix-store
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- -register-validity</command>. It is necessary to run this
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command on the remote machine after copying the inputs to inform
Nix on the remote machine that the inputs are valid
paths.</para></listitem>
</varlistentry>
</variablelist>
</para>
<para>The hook should copy the inputs to the remote machine,
register the validity of the inputs, perform the remote build, and
copy the outputs back to the local machine. An exit code other than
<literal>0</literal> indicates that the hook has failed. An exit
code equal to 100 means that the remote build failed (as opposed to,
e.g., a network error).</para>
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-->
</listitem>
</varlistentry>
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<varlistentry xml:id="envar-remote"><term><envar>NIX_REMOTE</envar></term>
<listitem><para>This variable should be set to
<literal>daemon</literal> if you want to use the Nix daemon to
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execute Nix operations. This is necessary in <link
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linkend="ssec-multi-user">multi-user Nix installations</link>.
Otherwise, it should be left unset.</para></listitem>
</varlistentry>
<varlistentry><term><envar>NIX_SHOW_STATS</envar></term>
<listitem><para>If set to <literal>1</literal>, Nix will print some
evaluation statistics, such as the number of values
allocated.</para></listitem>
</varlistentry>
<varlistentry><term><envar>NIX_COUNT_CALLS</envar></term>
<listitem><para>If set to <literal>1</literal>, Nix will print how
often functions were called during Nix expression evaluation. This
is useful for profiling your Nix expressions.</para></listitem>
</varlistentry>
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<varlistentry><term><envar>GC_INITIAL_HEAP_SIZE</envar></term>
<listitem><para>If Nix has been configured to use the Boehm garbage
collector, this variable sets the initial size of the heap in bytes.
It defaults to 384 MiB. Setting it to a low value reduces memory
consumption, but will increase runtime due to the overhead of
garbage collection.</para></listitem>
</varlistentry>
</variablelist>
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</chapter>